Photoluminescence from screen printed ZnO based nanocrystalline films

Photoluminescence from ZnO based films deposited by screen printing is studied. PL spectra of undoped films show strong broad green emission around 500 nm and a weak (nearly one fourth of visible) UV emission around 398 nm. Further enhancement of 30% in the green emission is observed as 5 at.% Ca is doped in ZnO which for higher dopant concentration falls monotonously. The films are polycrystalline and the particle size lies between 33 to 47 nm as determined by Debye‐Scherrer method. Film with 5 at.% doping is under tensile strain and the others are under compressive strain. SEM shows microclusters, consisting of nanoparticles, scattered throughout the substrate which club together with increase in dopant amount. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structural and optical properties of Mg‐doped ZnO thin films prepared by a modified Pechini method

ZnO thin films with different Mg doping contents (0%, 3%, 5%, 8%, 10%, respectively) were prepared on quartz glass substrates by a modified Pechini method. XRD patterns reveal that all the thin films possess a polycrystalline hexagonal wurtzite structure. The peak position of (002) plane for Mg‐doped ZnO thin films shifts toward higher angle due to the Mg doping. The crystallite size calculated by Debey‐Scherrer formula is in the range of 32.95–48.92 nm. The SEM images show that Mg‐doped ZnO thin films are composed of dense nanoparticles, and the thickness of Mg‐doped ZnO thin films with Mg doped at 8% is around 140 nm. The transmittance spectra indicate that Mg doping can increase the optical bandgap of ZnO thin films. The band gap is tailored from 3.36 eV to 3.66 eV by changing Mg doping concentration between 3% and 10%. The photoluminescence spectra show that the ultraviolet emission peak of Mg‐doped ZnO thin films shifts toward lower wavelength as Mg doping content increases from 3% to 8%. The green emission peak of Mg‐doped ZnO thin films with Mg doping contents were 3%, 8%, and 10% is attributed to the oxygen vacancies or donor‐acceptor pair. These results prove that Mg‐doped ZnO thin films based on a modified Pechini method have the potential applications in the optoelectronic devices.     

One‐step green synthesis by organic molten salt method and optical properties of CdS nanosheets

Cadmium sulfide (CdS) nanosheets were synthesized by an environment friendly, “green” organic molten salt (OMS) method at 220 °C. The as‐synthesized products were characterized by X‐ray diffraction (XRD), scanning electron microscopy (SEM), Transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS), respectively. The XRD results reveal that the as‐synthesized CdS nanosheets are of the hexagonal wurtzite structure and the CdS nanosheets grow along the c‐axis. The SEM results indicate that the diameters and thickness of the CdS nanosheets are about 20–40 nm and 5–10 nm, respectively. The optical properties of the CdS nanosheets were investigated by ultraviolet–visible (UV‐Vis) spectroscopy and photoluminescence (PL) spectroscopy. The ultraviolet–visible spectrum exhibits two excitonic peaks with a step‐like absorption and the photoluminescence spectrum shows a green emission peak centered at around 524 nm. A possible growth mechanism of CdS nanosheets was discussed.     

A direct suppression of green band emission of ZnO particles by adjusting the volume fraction of ethanol in reaction solutions

ZnO particles were successfully prepared by one step CTAB‐assisted hydrothermal method with different volume fraction of ethanol‐water mixture solution. The formed thorn‐ball like ZnO particles have an average size of 1 ∼ 2 μm in diameter. XRD result shows a hexagonal wurtzite structure and higher crystallinity. Room‐temperature photoluminescence shows a strong and dominated peak at ∼383 nm with a green emission at ∼510 nm. The intensity ratio between the UV and green emission increased from 1.31 to 7.53 when the volume fraction of ethanol was changed from 0% to 50%, which shows a direct suppression of structural defects just by adjusting the ethanol fraction in reaction solutions. The possible growth and luminescence mechanisms for thorn‐ball like ZnO particles are discussed.     

Glow curves and the emission of BaFCl:Eu2+

Thermoluminescence and the emission of γ-irradiated BaFCl:Eu²⁺ has been studied. A well defined glow peak at 400 K and a shoulder at 480 K are obtained in addition to those obtained as in undoped BaFCl. Thermoluminescence emission has shown a band at about 390 ∼ 400 nm. Additional glow peaks have been attributed to the luminescence centers as caused by europium impurity.     

Luminescence characteristics of undoped and Eu‐doped GdCa4O(BO3)3 single crystals and nanopowders

Single crystals of GdCa₄O(BO₃)₃ (GdCOB) pure and doped with Eu concentration of 1 and 4 at% were grown by the Czochralski and micropulling‐down methods. The distribution of Eu ions in GdCOB crystals was uniform. The substitutions of Eu³⁺ in Gd, Ca(1) and Ca(2) cation sites and eventually formation Eu²⁺ have been investigated. The spectroscopic properties of crystals are compared with the properties of nanopowders obtained by sol‐gel method. Radioluminescence spectra of undoped GdCOB crystal show the characteristic emission of Gd³⁺ at about 312 nm, whereas this emission dramatically decreases in Eu‐doped crystals upon X‐ray excitation, as well as in Eu‐doped nanopowders excited in vacuum ultraviolet (VUV) region. The VUV excitation in the range 125‐333 nm for Eu‐doped samples leads to strong emission in red coming from the ⁵D₀ multiplet of Eu³⁺, only. In the photoluminescence decay kinetics of 312 nm emissions substantial shortening and departure for single exponential decay in Eu‐doped samples is clearly observed. Higher Eu doping results in further acceleration of the decay. In undoped GdCOB crystal, the lifetime of the Gd^{3+ 6}P_7/2 multiplet is 2.79 ms. The Eu^{3+ 5}D₀ decay kinetics monitored at 613 nm are rather constant. Numerical fitting of fully exponential curves, reveals lifetimes 2.7 ms for nanopowder and 2.5 ms for single crystal. The results suggest that this material may be used as a red phosphor in plasma display panels in nanopowder form because of strong excitation band of Eu³⁺ luminescence in the 160‐200 nm regions. Contrary to nanopowder sample, such an excitation band, attributed to the Gd³⁺–O^2– charge transfer was not observed in crystal obtained by the micropulling‐down method. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Photoluminescence characteristics of Mg- and Si-doped GaN thin films grown by MOCVD technique

We have studied the optical, structural and surface morphology of doped and undoped GaN thin films. The p- and n-type thin films have been successfully prepared by low-pressure MOCVD technique by doping with Mg and Si, respectively. The different carrier concentrations were obtained in the GaN thin films by varying dopant concentrations. Photoluminescence (PL) studies were carried to find the defect levels in the doped and undoped GaN thin films at low temperature. In the undoped GaN thin films, a low intensity and broad yellow band peak was observed. The donor–acceptor pair (DAP) emission and its phonon replicas were observed in both the Si or Mg lightly doped GaN thin films. The dominance of the blue and the yellow emissions increased in the PL spectra, as the carrier concentration was increased. The XRD and SEM analyses were employed to study the structural and surface morphology of the films, respectively. Both the doped and the undoped films exhibited hexagonal structure and polycrystalline nature. Mg-doped GaN thin films showed columnar structure whereas Si-doped films exhibited spherical shape grains.     

Structural,optical and photoluminescence properties of ZnS: Cu nanoparticle thin films as a function of dopant concentration and quantum confinement effect

Thin films of ZnS: Cu nanoparticles were deposited in chemical bath by a pH controlled solution synthesis technique. The copper concentration was varied from 0 to 0.1M%. XRD and SEM indicated variations in diffracted intensity and morphology with Cu concentration. The PL spectrum of the undoped ZnS nanoparticles showed emission peaks at 393 and 432nm that could be attributed to the intrinsic defect states of ZnS nanoparticles. For ZnS: Cu samples three peaks in the range of 390nm, 480nm and 525nm were observed. With increase in Cu concentration from 0.001 to 0.1M%, the peak position of 480nm and 525nm did not change, whereas 390nm peak red shifted to longer wavelength region to 422nm. In addition, it was found that the overall photoluminescence intensity reached maximum at 0.01M% and quenched with further increase in Cu concentration. Enhancement of blue and green light emission by seven and twenty fivefold respectively compared to undoped ZnS was observed in ZnS: Cu with optimal dopant concentration. Time resolved decay of photoluminescence showed faster decay for 390 – 420nm purple/ blue emission compared to green (525nm) Cu related emission which is in the microsecond time scale. Optical absorption measurements indicate enhancement of band gap (3.89eV) for undoped ZnS suggesting the quantum confinement effect in the developed nanoparticles of size below the Bohr diameter. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Crystallographic,optical and magnetic properties of Co‐doped ZnO thin films synthesized by sol gel route

ZnO thin films with various Co doping levels (0%, 1%, 3%, 5%, 8%, respectively) have been synthesized by sol gel spin coating method on glass substrates. XRD and XPS studies of the films reveal that cobalt ions are successfully doped into ZnO crystal lattice without changing the hexagonal wurtzite structure. The morphologies are studied by SEM and AFM and show wrinkle network structures with uniform size distribution. With Co doping concentration increasing, the wrinkle network width decreases gradually. The transmittance spectra indicate that Co doping can effectively reduce the optical bandgap of ZnO thin films. Photoluminescence show that all samples have ultraviolet, violet and green emission. When Co doping concentration increases up to 5%, the intensity of violet emission is greatly increased and a strong deep blue emission centered at 439 nm appears. The ferromagnetism of all samples was observed at room temperature. The emission mechanisms and ferromagnetism origination are discussed in detail. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Preparation of dendritic‐like CdS by hydrothermal method and its photocatalytic performance

In this article, dendritic‐like CdS has been prepared by a hydrothermal method using thiourea as the sulfur source, and the effects of experimental conditions on the morphologies of CdS have been investigated. The performances of CdS have been analyzed by X‐ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and the fluorescence and photodegradation properties of CdS have also been investigated. The XRD result indicates that the dendritic‐like CdS are of hexagonal phase and they are highly crystallized. Also, the FESEM results show that the ratio of raw material affects the yield of CdS, the reaction time affects the morphology of CdS. The best morphology of CdS is dendritic structures and the length is about 6 μm. The fluorescence spectrum shows three peaks at 470 nm, 513 nm and 547 nm, which indicates that the dendritic‐like CdS mainly emits green and blue fluorescence. Moreover, the dendritic‐like CdS exhibits good photocatalytic activity and its photodegradation rate to methylene blue can reach 92%. The growth mechanism for the formation of CdS with dendritic structure is also described.     

Ammonia-free chemical bath deposition of CdS films: tailoring the nanocrystal sizes

We report on the control of nanocrystal sizes in CdS nanocrystalline films prepared by ammonia-free chemical bath deposition technique. We studied the effect of deposition duration, bath temperature during deposition, and post-preparation heat treatment. Nanocrystals (NCs) with radii from 2.6 nm to more than 10 nm were prepared and characterised by transmission electron microscopy, X-ray diffraction, and optical photoluminescence and absorption spectroscopy. We observed cubic to hexagonal phase transition for large NCs prepared by the heat treatment.     

Optical properties of flower‐like CdS prepared by a hydrothermal method with SDBS as surfactant

In this article, flower‐like CdS structures have been prepared by a hydrothermal method with SDBS as surfactant. The influences of different experimental conditions on the morphologies, UV‐Vis and fluorescence properties of CdS have been investigated. The performances of CdS have been analyzed by X‐ray diffraction (XRD), field emission scanning electron microscopy (FESEM), ultraviolet‐visible (UV–Vis) and room‐temperature photoluminescence (PL). The XRD result indicates that the flower‐like CdS structures are of hexagonal phase. The FESEM results indicate that the main role of SDBS is to make the CdS crystals assemble together to form the flower‐like structures. The UV–Vis results show CdS has a strong absorption in the ultraviolet region and visible‐light region. The PL results show CdS has two emission peaks, respectively at 461 nm and 553 nm. The growth mechanism for the formation of flower‐like CdS structures is also described.     

Crystallography and cathodoluminescence of ultra‐long GaN nanowires

Ultra‐long GaN nanowires have been synthesized via a simple thermal evaporation process by heating mixed GaN and Ga₂O₃ powders in a conventional resistance furnace under ammonia gas at 1150 °C. The average length of GaN nanowires is estimated to be more than 100 μm after 30‐min growth, corresponding to a fast growth rate of more than 200 μm/h. Scanning electron microscope (SEM) observation indicated that the diameter of GaN nanowires was rather uniform along the growth direction and in the range of 100–200 nm. X‐ray diffraction (XRD) and transmission electron microscope (TEM) measurements confirmed that the GaN nanowires are crystalline wurtzite‐type hexagonal structure. Room‐temperature cathodoluminescence (CL) measurement indicated that an obvious red‐shift of the near band‐edge emission peak centered at 414 nm of the ultra‐long GaN nanowires and a wide shoulder in the range of 600–700 nm were observed. Possible reasons responsible for the red‐shift of the near band‐edge emission of the ultra‐long GaN nanowires was discussed. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis and optical properties of Ce-doped ZnO hexagonal nanoplatelets

Single crystalline Ce-doped ZnO hexagonal nanoplatelets are successfully synthesized. Zinc acetate, cerium nitrate, potassium hydroxide and poly vinyl alcohol were mixed together and transferred to a 100 mL Teflon-lined stainless steel autoclave kept at 150 °C for 24 h. The obtained precipitant is calcined at 600 °C. The morphology and microstructure were determined by field emission scanning electron microscopy (FE-SEM), X-ray diffraction transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX) and photoluminescence (PL) spectroscopy. The investigation confirmed that the products were of the wurtzite structure of ZnO. The doped hexagonal nanoplatelets have edge length 25 nm and thickness 11 nm. EDX result showed that the amount of Ce in the product is about 15%. Photoluminescence of these doped hexagonal nanoplatelets exhibits a blue shift and weak ultraviolet (UV) emission peak, compared with pure ZnO, which may be induced by Ce-doping. The growth mechanism of the doped hexagonal nanoplatelets was also discussed.     

Raman spectroscopic investigations of Mn2+ doping effects on the densification of acid-catalyzed silica xerogels

Undoped and Mn²⁺-doped silica xerogels were prepared from hydrolysis and condensation of tetramethyl orthosilicate (TMOS). The xerogels were characterised by density measurements and fluorescence and Raman spectroscopies. Raman measurements over the range 4–1200 cm⁻¹ showed that the number of three- and four-membered rings in the xerogel network depends on the thermal treatment and on the concentration of Mn²⁺ ions. Indeed, both structures are found to be more numerous in the gel network of the doped samples than in the undoped one, showing that doping with Mn²⁺ hampers the destruction of three- and four-membered rings. In the low-wave number region (4–100 cm⁻¹), doping with manganese ions was found to affect the position of the boson peak. The boson peak profiles were used to deduce that the sizes of the cohesive domains in the gel-derived silica network are much larger for doped samples (11 nm for 500 ppm) than for undoped ones (2.1 nm).     

Influence of doping on OH absorption in LiNbO3 crystals

Undoped, Cr doped and Mg, Cr codoped LiNbO₃ crystals were grown by conventional Czochralski technique. Comparative study was carried out using Fourier transform infrared (FTIR) and UV‐Visible spectroscopy. Infrared optical absorption for OH^– ion has been used to study the effect of dopants on the crystals. The peak position of OH^– shift to 3535 cm^‐1 for Mg, Cr codoped crystals compared to 3484 cm^‐1 for undoped and Cr doped crystals. Prominent absorption bands are found in the visible region centered at 480 nm (20833 cm^‐1) and 653 nm (15313 cm^‐1) in Cr doped crystals. Whereas in Mg, Cr codoped crystals these broad absorption bands are red shifted to 517 nm (19342 cm^‐1) and 678 nm (14749 cm^‐1). UV cutoff in Cr doped crystals shift towards higher wavelength compared to undoped LiNbO₃ crystals. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Material Characterization of Chemically Deposited Photoconducting CdS Films

Results of SEM, XRD and absorption measurements were obtained to characterize the chemically deposited undoped and rare earth doped CdS films. SEM studies present the crystalline nature of undoped CdS. In Presence of NaF layered growth takes place which finally results in cabbage type growth. In presence of rare earths cabbage and crystalline growth are observed. XRD studies of undoped CdS show combination of cubic as well as hexagonal phases. Presence of impurities results in change in itensity of peaks along with appearance of some new peaks. The absorption coefficient studies show a band gap of 2.41 eV for undoped CdS.     

Spray deposition and characterization of undoped and In‐doped tin disulphide thin films

Undoped and Indium doped tin disulphide (SnS₂) thin films had been deposited onto glass substrates at T_s = 300 °C using spray pyrolysis technique under atmospheric pressure with stannous chloride, indium chloride and thiourea as precursors. The structural, optical and electrical properties of the deposited films were characterized. The XRD pattern revealed that the undoped and doped films had preferred orientation along (002) plane with hexagonal structure. FESEM micrographs had shown that morphologies of the films changed with indium doping. Optical constant such as refractive index (n), extinction coefficient (k), real and imaginary parts of dielectric constants were evaluated from transmittance and reflectance spectra in UV‐Visible spectroscopy. The optical absorption data were used to determine the band gap energy and it was found to be 2.75 eV for undoped and 2.50 eV for indium doped films respectively. The room temperature dark resistivity was found to be 4.545 × 10³ Ω‐cm and 5.406 × 10³ Ω‐cm for undoped and In‐doped films respectively.     

Cd‐Doping Effects on The Properties of Polycrystalline α‐In2Se3 Thin Films

The X‐ray diffraction has revealed that the polycrystalline hexagonal structured α‐In₂Se₃ thin films grown at substrate temperature of 200 °C with the unit cell parameters a = 4.03 Å and c = 19.23 Å becomes polycrystalline hexagonal structured InSe with a unit cell parameters of a = 4.00 Å and c = 16.63 Å by Cd‐doping. The analysis of the conductivity temperature dependence in the range 300‐40 K revealed that the thermionic emission of charged carriers and the variable range hopping are the predominant conduction mechanism above and below 100 K, respectively. Hall measurements revealed that the mobility is limited by the scattering of charged carriers through the grain boundaries above 200 K and 120 K for the undoped and Cd‐doped samples, respectively. The photocurrent (I_ph) increases with increasing illumination intensity (F) and decreasing temperature up to a maximum temperature of ∼100 K, below which I_ph is temperature invariant. It is found to have the monomolecular and bimolecular recombination characters at low and high illumination intensities, respectively. The Cd‐doping increases the density of trapping states that changes the position of the dark Fermi level leading to the deviation from linearity in the dependence of I_ph on F at low illumination intensities.     

Homoepitaxial growth and photoluminescence of self‐assembled In‐doped ZnS nanowire bundles

Self‐assembled In (Indium)‐doped ZnS nanowire bundles were synthesized via a thermal evaporation method without using any template. Vapor ‐ solid homoepitaxial growth was found to be the key reason for the formation of close‐packed nanowire bundles grown on the surface of microscale sphere‐shaped ZnS crystal. X‐ray diffraction (XRD), selected area electron diffraction (SAED), and transmission electron microscopy (TEM) analysis demonstrate that the In‐doped ZnS nanowires have the cubic structure, and there are numerous stacking faults along the <111> direction. Photoluminescence (PL) spectrum shows that the spectrum mainly includes two parts: a weak violet emission band centering at about 380 nm and a strong green emission band centering at about 510 nm. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)